1. Field of the Invention
The present invention concerns the field of micromachining semiconductor substrates for the production of micro-electro-mechanical systems (MEMS) or micro-opto-electro-mechanical systems (MOEMS). It relates more particularly to the control and to the regulation of the gas pressure in the process chamber during the different steps of the fabrication of semiconductor components by the alternating plasma etching technique, in particular to the process utilized.
2. Description of the Prior Art
In industrial processes for fabricating or processing products in enclosures supplied with processing gas at very low pressures, it is necessary to regulate the pressure in the enclosure. This is the case, for example, in methods of fabricating or processing semiconductors and micro-electro-mechanical systems (MEMS).
The substrate is placed in a vacuum enclosure containing active gases that are ionized by plasma generation means. The processing generally comprises successive steps that take place in a process chamber in a low-pressure atmosphere enabling a plasma to be established and maintained.
For certain applications, processes employing alternating flows of different gases are used. For example in the case of etching silicon, it is common practice to use a succession of cycles each comprising two steps: an etching step comprising the introduction of a gas or a gas mixture containing fluorine, such as SF6, and a step of deposition of a passivation film onto the surfaces comprising the introduction of a gas CxFy, such as C4F8 for example. One or several supplementary steps comprising the introduction of a neutral or cleaning gas may be added. Each step includes the simultaneous injection of gas into the process chamber and for each step there is defined a constant gas flow rate. However, the nature and the flow rate of the gas vary from one step to another, which causes a variation of the pressure in the process chamber in each step. That pressure is an important parameter for the control and reproducibility of the performance of the process step. Thus each step is characterized by a gas pressure in the process chamber that it is necessary to control and to regulate optimally.
The appropriate vacuum necessary for the different process steps is generated and maintained by a vacuum line comprising vacuum pumps connected to the process chamber. It is possible to adjust the pressure in the process chamber using a throttle valve disposed at the inlet of the pumping line. However, the duration of each step being variable and relatively short (a few milliseconds to a few seconds), the response time of the valve prevents accurate control of the pressure. The choice is therefore usually made to impose on the valve a position that is constant throughout the treatment and that makes it possible to control the pressure approximately.
A variation of pressure in time is also observed for the same process. It is known that, for the same position of the valve, the pressure in the process chamber can vary according to the state of the reactor, all other things being equal. The pressure drift observed may be due to a modification of the state of cleanliness of the reactor, for example because of the deposition of products of degradation generated during the treatment. This variation depends also on the type of throttle valve. Consequently, it is necessary to define for each installation, and as a function of the evolution of the condition of the reactor, a different position of the valve for each of the steps of the treatment effected.
The document WO-2004/093165 proposes a method of controlling the pressure during the alternation of etching and passivation steps. The method consists in, at the beginning of the treatment, fixing the position of the valve connecting the process chamber to the turbo pump independently for each step. The starting position of the valve is determined as a function of data acquired during one or more earlier steps of the same type (this is called “position” mode control). After at least a portion of the step in that position, the pressure is then regulated by varying the position of the valve by means of a control algorithm (this is called “pressure” mode control).
As the pressure in the process chamber is an important parameter for the control and the reproducibility of the performance of the plasma etching treatment, it is necessary to regulate that parameter precisely. An object of the present invention is to minimize the pressure drift during the treatment by proposing a method of controlling and regulating the pressure in a process chamber.